Prior to the present invention in the manufacture of silver-cadmium oxide contacts by powder metallurgy, the Ag-CdO powder mix is pressed together with pure silver powder or a mixture of silver powder and a thermally decomposable silver compound to form a two layer composite structure consisting of the Ag-CdO body and a thin backing layer of fine silver. The fine silver backing is required to facilitate bonding of the contact to a support member by a brazing technique. Without the backing layer, the presence of CdO on the brazing surface causes poor wetting of braze metal due to a reaction between CdO and the molten braze metal. In one older method pure silver powder is filled into the pressing die cavity partially filled with the AgCdO mix and these are pressed together. The double layered compact sinters to a non-flat configuration, requiring a subsequent coining or shaping operation. Thermally coarsened silver powder, often used to facilitate powder flow, further aggravates the warpage condition because the thermal coarsening further reduces shrinkage of the silver powder layer relative to Ag-CdO body. However, the use of fine silver powder in the double-fill compacting operation causes warpage of the part during sintering due to a difference in shrinkage rate between Ag-CdO and fine silver. The dissimilar shrinkage arises from the fact that silver powder is compressed to a higher green density than the Ag-CdO mix in the double-fill pressing as a result of a higher compressibility of unmixed silver. During sintering, the higher green density silver layer shrinks less than the Ag-CdO body, causing warpage or bending. Such non-flat condition causes difficulty during subsequent coining to a final contact shape, but more importantly, contributes to part cracking; the severity of which depends on CdO content and the degree of coining needed for final configuration. The coining crack, however minute it may be, is one of the causes of excess erosion leading to early contact failure. Due to tremendous thermal stress generation during arcing and cooling, the crack slowly opens up and propagates through the contact body causing complete separation of a piece of contact. Another older method utilizes mixture of silver powder and a thermally decomposable silver compound, namely silver oxide or silver carbonate, as a backing component in the double-fill press operation. The presence of the decomposable silver compound generates additional porosity during sintering, resulting in additional shrinkage, thereby reducing warpage of the part. Complete elimination of warpage is, however, difficult to achieve since the amount of the poregenerating compound is limited to a certain fraction of the mix. This limitation is dictated by the fact that pressing laminations, blisters, and less than optimum density result when too high a compacting pressure is used in combination with high silver oxide or carbonate content.
U.S. Pat. No. 3,510,935 discloses the use of mixtures of silver and silver carbonate and data is given to show the superiority of these mixtures over silver. In addition, that patent mentions that other gas-evolving, thermally decomposable silver compounds such as silver oxide, silver acetate, silver nitrate and silver oxalate are equivalents of silver carbonate, however, no data is given in regard to the use of any such materials other than silver carbonate.
Recently attempts have been made to reduce the uneven shrinkage of the double layered powder metal contact by using a silver powder mixture for the backing layer, which has a shrinkage characteristic similar to that of the Ag-CdO contact body. This was achieved by incorporating silver acetate, which has high inherent shrinkage, into the silver powder.
The advantage of pressing and sintering a mixture of pure silver and low density silver acetate rather than pure silver and the higher density salts such as silver oxide or silver carbonate to obtain a compatible, structurally sound backing layer has been demonstrated. It is also been found that compatibility with the bulk and as-sintered density improves with increasing silver acetate content up to about 30 wt% silver acetate-balance pure silver. However, with increasing silver acetate content above about 15 wt%, even though the bulk density increases, the distribution of the remaining pore volume changes from a microscopic condition to very small rather spherical macroscopic voids. It is believed that this condition is a result of incipient melting of silver acetate caused by the inhibited ventilation of volatiles from the dense pressed compact during heating. Obviously, the greater the silver acetate content, the greater the probability of this condition occurring.
It is believed, therefore, that it is an advancement in the art to provide a contact having a silver backing relatively free of warpage and relatively free of macroscopic voids.